1. Field of the Invention
The present invention relates to a folded dipole antenna and a tag using the same, and in particular to a noncontact folded dipole antenna for a signal transmission/reception to/from an RFID reader/writer, and an RFID tag using the same.
2. Description of the Related Art
An RFID system has been already known in which a reader/writer transmits a signal of approximately 1 W via a radio line of a UHF bandwidth (860-960 MHz), and a tag receives the signal and returns a response signal to the reader/writer, thereby enabling information within the tag to be read by the reader/writer. It is stipulated that the communication frequency is 953 MHz, whereby the communication distance is approximately 3 m, while it depends on the gain of an antenna provided on the tag and the operation voltage and a peripheral environment of a chip. The tag is composed of an antenna approximately 0.1 mm thick and an LSI chip (whose size is approximately 1 mm square and 0.2 mm thick) connected to an antenna feeding portion.
As shown in FIG. 8, an LSI chip 21 can be equivalently represented by a parallel circuit of an internal resistance Rc (e.g. 1200 Ω) and a capacitance Cc (e.g. 0.7 pF). An admittance Yc (=1/Rc+jwCc) of the chip 21 is indicated at a position A21 on an admittance chart of FIG. 9. On the other hand, an antenna 22 can be equivalently represented by a parallel circuit of a radiation resistance Ra (e.g. 500 Ω) and an inductance La (e.g. 40 nH).
By connecting the chip 21 to the antenna 22 in parallel, the capacitance Cc and the inductance La resonate with each other and make impedance matching at a desired resonant frequency fo (the above-mentioned 953 MHz), so that the maximum reception power at the antenna 22 is supplied to the chip 21, as seen from the following equation.
                    fo        =                  1                      2            ⁢                                                  ⁢            π            ⁢                                                  ⁢                          LC                                                          Eq        .                                  ⁢                  (          1          )                    
As a basic antenna used for an RFID tag, a dipole antenna 31 approximately 145 mm (λ/2) long shown in FIG. 10A can be mentioned. The impedance in this case plots a track (1) in FIG. 9. At fo=953 MHz, Ra assumes 72 Ω and the imaginary part assumes 0, which are indicated at a position A31 on the track (1).
Since the radiation resistance Ra required for the antenna of the RFID tag is as extremely high as approximately 500-2000 Ω, the radiation resistance Ra is required to be raised from 72 Ω.
It is well known that with a folded dipole antenna 32 approximately 145 mm long as shown in FIG. 10B the radiation resistance Ra is raised from 72 Ω of the dipole antenna to approximately 300 Ω-500 Ω, depending on a line width (see e.g. non-patent document 1).
FIG. 9 shows that the impedance of the folded dipole antenna 32 plots a track (2), and at fo=953 MHz, Ra assumes 500 Ω and the imaginary part assumes 0, which are indicated at a position A32 on the track (2).
Furthermore, by connecting an inductance portion 33 in parallel to the folded dipole antenna 32 shown in FIG. 10B as shown in FIG. 10C, the track (2) on the admittance chart of FIG. 11 is rotated counterclockwise, so that the impedance can be indicated at a position A33 on the track (3) with an imaginary component (Ba=−1/ωLa) of the same absolute value as the imaginary component (Bc=ωCc) of the admittance of the chip 21. In this case, the shorter the length of the inductance portion 33 becomes, the smaller the value of the inductance La becomes, which leads to a large imaginary component and a large rotation amount.
Since the imaginary component Bc of the chip 21 has the same magnitude as that of the imaginary component Ba of the antenna 22, they are cancelled mutually and the resonance occurs at the frequency fo. The canceling of the imaginary components is the most important element upon designing an RFID tag. Although matching between the internal resistance Rc of the chip 21 and the radiation resistance Ra of the antenna 22 is the most preferable, it is not necessary to strictly match them with each other.
On the other hand, there is a radio tag operating in two frequency bands by arranging a non-feeding element of a half wavelength resonating in 2.4 GHz band formed by a conductive pattern on the opposite side of a folded dipole antenna across a dielectric sheet at the folded dipole antenna resonating in 900 MHz band formed by the conductive pattern on the dielectric sheet, and by performing impedance matching for two frequency bands (see e.g. patent document 1).
[Non-patent document 1] Antenna engineering handbook: Page 112 (published on Mar. 5, 1999 by Ohmsha)
[Patent document 1] Japanese Patent Application Laid-open No. 2005-236468
FIG. 11 shows an arrangement of the above-mentioned RFID system. An R/W-end antenna 13 connected to a reader/writer (R/W) 11 through a cable 12 is a patch antenna or the like having a circularly-polarized wave characteristic. Since an electric field direction “A” from the reader/writer 11 is always rotated as shown in FIG. 11, a tag 15 with an antenna which is generally a linearly-polarized wave can transmit/receive a signal to/from the reader/writer 11 through a radio wave propagation path 14, whichever direction the tag 15 faces.
When the folded dipole antenna shown in FIG. 10B is used for the tag 15, the folded dipole antenna also has a linearly-polarized wave characteristic. Therefore, if an appropriate electric field can be generated in a surface orthogonal to a linearly-polarized wave surface specific to the folded dipole antenna, a polarized wave orthogonal to the electric field direction from the reader/writer 11 at a certain point can be received, and the communication distance of the tag 15 can be extended.
Although there has been known a cross dipole as shown in FIG. 12, it makes the tag too huge for the practical use.